1. Field of the Invention
The present invention relates to an induction motor and, more particularly, to an induction motor equipped with a heat dissipating disc that dissipates heat generated from the rotor.
2. Description of the Related Art
Induction motors have a simple structure and thereby can be easily serviced, so they are most widely used in the industry. The induction motor has independent windings in its stator and rotor; it is operated by electromagnetic induction caused from one winding to the other winding.
FIG. 6 is a perspective view showing an example of a conventional induction motor 100. FIG. 7 is a cross sectional view of the conventional induction motor shown in FIG. 6. The induction motor 100 has a stator 1 and a rotor 2; end brackets 31, 32 are attached at both ends of the stator 1, and the rotor 2, which is cylindrical, is fixed to a first shaft 3 rotating about a rotation center axis 22 of the induction motor, the rotor 2 being inserted inside the inner circumference of the stator 1.
The first shaft 3, to which the rotor 2 is fixed, is rotatably supported by a bearing 23 attached to the end bracket 31 and by a bearing 25 attached to the end bracket 32. A code plate, which is part of a rotation detector 4, is fixed to the first shaft 3 at the end of the unloaded side, concentrically with the rotation center axis 22.
A plurality of slots is formed in the inner wall of the stator 1, along a direction parallel to the rotation center axis 22, a stator winding being placed in these slots. Part of the stator winding extends beyond both ends of the stator 1 as coil ends 24 and 26.
Due to the principle of operation of the induction motor 100, the rotor 2 generates heat in proportion to generated torque. That is, the surfaces of the rotor 2 become hot due to a loss caused by a polarity change. When the rotation detector 4 is attached to the induction motor 100, the heat generated from the rotor 2 may cause adverse effects such as damage to the rotation detector 4 and a reduction in detection precision.
As shown in FIG. 7, a cooling fan 6 is provided in a first fan housing 5 so that it faces the end bracket 32 in which the rotation detector 4 is disposed. The cooling fan 6 sucks outside air from cooling air inlets 27 through vents 7 formed in the end brackets 31, 32 and the stator 1 and exhausts the sucked air through cooling air outlets 28, so the cooling fan 6 indirectly cools through a lid 30 the first shaft 3 to which the rotation detector 4 is attached. Accordingly, a cooling air flow 8 does not strike the rotor 2 or first shaft 3 and thus the rotor 2 becomes hot, so the rotation detector 4 attached to the shaft is thermally damaged and detection precision drops.
There is a known technology by which the heat generated from the rotor 2 is dissipated by a heat dissipating in attached to the rotor 2.
Japanese Patent Application Laid-Open No. 2008-43149 discloses a technology in which a dissipating fin is attached to a rotor. With this technology, outside air is taken into the interior of a motor, so the bearings and windings of the motor are highly likely to be damaged in an environment filled with dust or an atomized liquid.
Japanese Patent Application Laid-Open No. 2000-32710 discloses another technology in which a relay axis is attached to a rotor so as to extend toward an unloaded side, and a heat dissipating fin is attached between a rotation detector and the rotor. Since, in this technology, the rotation detector is disposed at a location distant from a bearing, the natural frequency of the relay axis is reduced and thereby the rotational speed is limited and detection precision drops.
The technologies disclosed in the above two patent documents have a common problem in that, although the cooling power needs to be maximized while the motor is operating with large torque at low speed, the cooling power cannot be fully used.